Discusion API now takes a view parameter when getting a thread list.
This view parameter only takes "unread" or "unanswered" as possible
values.

Flagging/unflagging is done by issuing a PATCH request on a thread or
comment endpoint with the "abuse_flagged" field set.

This implements the "following" value for the "view" query parameter on
the thread list endpoint.

This will allow clients to determine what actions (e.g. following,
endorsing, and editing content) a user can take on a piece of content.

The thread list endpoint now accepts the text_search parameter and also
includes a text_search_rewrite field in its responses.

This is done via PATCH on a comment instance endpoint.

This depends on cs_comments_service commit 0487891.

This is done via PATCH on a thread instance endpoint.

This allows authors to follow the thread immediately upon creation.

Also, fix the field-specific error format for all API endpoints.

This requires cs_comments_service commit fdf017c918.

This commit adds fields that are related to the requesting user's
interaction with the thread (e.g. following).

This is an initial implementation that only allows retrieval of all
threads for a course and only returns an easily computed subset of the
fields that are needed, in order to keep this change from getting too
large.

JIRA: MA-641